This application claims the benefit of Korean Application Nos. P2001-32410 filed on Jun. 11, 2001 and P2001-54327 filed on Sep. 5, 2001, which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a method and apparatus for driving a liquid crystal display. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing deterioration in picture quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell in accordance with a video signal, thereby displaying a picture. An active matrix LCD including a switching device for each liquid crystal cell is suitable for displaying a moving picture. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
The LCD has a disadvantage in that it has a slow response time due to inherent characteristics of a liquid crystal such as a viscosity and an elasticity, etc. Such characteristics can be explained by using the following equations (1) and (2):
xe2x80x83xcfx84rxe2x88x9dxcex3d2/xcex94xcex5|Va2xe2x88x92VF2|xe2x80x83xe2x80x83(1)
where xcfx84r represents a rising time when a voltage is applied to a liquid crystal, Va is an applied voltage, VF represents a Freederick transition voltage at which liquid crystal molecules begin to perform an inclined motion, d is a cell gap of liquid crystal cells, and xcex3 represents a rotational viscosity of the liquid crystal molecules.
xcfx84f=xcex3d2/Kxe2x80x83xe2x80x83(2)
where xcfx84f represents a falling time at which a liquid crystal is returned into the initial position by an elastic restoring force after a voltage applied to the liquid crystal was turned off, and K is an elastic constant.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of a liquid crystal and a cell gap, etc. Typically, the TN mode liquid crystal has a rising time of 20 to 80 ms and a falling time of 20 to 30 ms. Since such a liquid crystal has a response time longer than one frame interval (i.e., 16.67 ms in the case of NTSC system) of a moving picture, a voltage charged in the liquid crystal cell is progressed into the next frame prior to arriving at a target voltage. Thus, due to a motion-blurring phenomenon, a moving picture is blurred out on the screen.
Referring to FIG. 1, the conventional LCD cannot express desired color and brightness. Upon implementation of a moving picture, a display brightness BL fails to arrive at a target brightness corresponding to a change of the video data VD from one level to another level due to its slow response time. Accordingly, a motion-blurring phenomenon appears from the moving picture and a display quality is deteriorated in the LCD due to a reduction in a contrast ratio.
In order to overcome such a slow response time of the LCD, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO99/05567 have suggested to modulate data in accordance with a difference in data by using a look-up table (hereinafter referred to as high-speed driving scheme). This high-speed driving scheme allows data to be modulated by a principle as shown in FIG. 2.
Referring to FIG. 2, a conventional high-speed driving scheme modulates input data VD and applies the modulated data MVD to the liquid crystal cell, thereby obtaining a desired brightness MBL. In the high-speed driving scheme, |Va2xe2x88x92VF2| is increased from the above equation (1) on the basis of a difference of the data so that a desired brightness can be obtained in response to a brightness value of the input data within one frame interval, thereby rapidly reducing a response time of the liquid crystal. Accordingly, the LCD employing such a high-speed driving scheme compensates for a slow response time of the liquid crystal by modulating the data value in order to alleviate a motion-blurring phenomenon in a moving picture, thereby displaying a picture at desired color and brightness.
Referring to FIG. 3, a conventional high-speed driving apparatus includes a frame memory 33 connected to a most significant bit output bus line 32, and a look-up table 34 connected to the most significant bit output bus line 32 and the frame memory 33.
The frame memory 33 stores most significant bit data MSB during one frame interval and supplies the stored data to the look-up table 34. Herein, the most significant bit data MSB can be set to high-order 3 or 4 bits, but may be set up to 5 or 6 bits if needed.
The look-up table 34 is a mapping of most significant bit data of a current frame Fn inputted from the most significant bit output bus line 32 and most significant bit data of the previous frame Fnxe2x88x921 inputted from the frame memory 33 into a modulation data table as shown in Table 1, thereby outputting modulated data Mdata. Such modulated most significant bit data Mdata are added to non-modulated least significant bit data.
When the most significant bit data MSB are limited to 4 bits, a look-up table in the high-speed driving scheme is implemented by the following tables:
In the above tables, a furthermost left column is for a data voltage VDnxe2x88x921 of the previous frame Fnxe2x88x921 while an uppermost row is for a data voltage VDn of the current frame Fn. Table 1 is look-up table information in which the most significant bits (i.e., 20, 21, 22 and 23) are expressed by the decimal number at. Table 2 is look-up table information in which weighting values (i.e., 24 25, 26 and 27) of the most significant 4 bits are applied to 8-bit data.
If the most significant bit data MSB are configured by 4 bits and the most significant bit data MSB of the previous frame Fnxe2x88x921 and the most significant bit data MSB of the current frame Fn are given as shown in FIG. 4, the data Mdata modulated by the look-up table 34 become larger than the most significant bit data MSB of the current frame Fn.
However, the conventional high-speed driving apparatus has a problem in that data values of the modulated data Mdata become excessively larger than a real change amount of the data even when data values of the previous frame Fnxe2x88x921 and the current frame Fn are slightly changed.
Referring to FIG. 5, a data gray level value xe2x80x9800011111xe2x80x99 of the previous frame Fnxe2x88x921 is changed into xe2x80x9800100000xe2x80x99 at the current frame Fn. If a modulation is made by a modulation table such as Table 1 for a high-speed driving scheme, values of the most significant bit data are increased to xe2x80x9800110000xe2x80x99. In this process, a real gray level value difference between the previous frame Fnxe2x88x921 and the current frame Fn is only the decimal number xe2x80x981xe2x80x99. In other words, a gray level value xe2x80x9831xe2x80x99 of the previous frame Fnxe2x88x921 is slightly changed into xe2x80x9832xe2x80x99 at the current frame Fn. However, a value modulated by the modulation table using Table 1 becomes xe2x80x9848xe2x80x99. Thus, a picture having gray levels with a slight change becomes to have a gray level difference of xe2x80x9817xe2x80x99 due to the data modulation in the conventional high-speed driving scheme.
If a modulation is made with a value largely different from a real gray level value, an undesirably excessive voltage is applied to the liquid crystal cell. Thus, an unnecessary bright stripe is emerged from the boundary portion where the data is changed.
Accordingly, the present invention is directed to a method and apparatus for driving a liquid crystal display that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide a method and apparatus for driving a liquid crystal display that is adaptive for preventing a deterioration of picture quality.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of driving a liquid crystal display includes dividing input data into most significant bit data and least significant bit data, detecting changes in the most significant bit data and the least significant bit data, comparing the change in the most significant bit data with a first reference value, comparing the change in the least significant bit data with a second reference value, and determining whether input data are modulated in accordance with the compared results.
In the method, the detecting changes includes delaying the most significant bit data and the least significant bit data for a frame period, calculating a first difference between non-delayed most significant bit data and the delayed most significant bit data, and calculating a second difference between non-delayed least significant bit data and the delayed least significant bit data.
In another aspect of the present invention, a method of driving a liquid crystal display includes dividing input data from an input line into most significant bit data and least significant bit data, detecting changes in the number of bits in the most significant bit data and the least significant bit data between current and previous frames, comparing the change in the number of bits of the most significant bit data with a first reference value, comparing the change in the least significant bit data with a second reference value, modulating the input data if the change in the most significant bit data is equal to the first reference value and the change in the least significant bit data is smaller than the second reference value, and supplying the input data without a modulation if the change in the most significant bit data is equal to the first reference value and the change in the least significant bit data is equal to or larger than the second reference value.
The method further includes modulating the input data if the change in the most significant bit data is different from the first reference value.
In another aspect of the present invention, a method of driving a liquid crystal display includes dividing input data from an input line into most significant bit data and least significant bit data, determining whether current most significant bit data are included in a first data area of a look-up table, comparing a first difference value in the number of bits between current least significant bit data and previous least significant bit data with a first reference value if the current most significant bit data are included in the first data area of the look-up table, determining whether the current most significant bit data are to be modulated in accordance with the compared result of the first difference value, determining whether the current most significant bit data are included in a second data area of a look-up table, comparing a second difference value in the number of bits between the current input least significant bit data and the previous least significant bit data with a second reference value if the current most significant bit data are included in the second data area of the look-up table, and determining whether the current most significant bit data are to be modulated in accordance with the compared result of the second difference value.
In the method, the determining whether current most significant bit data are included in a first data area of a look-up table includes subtracting the current most significant bit data from the previous most significant bit data if the number of bits of the current most significant bit is greater than that of the previous most significant bit data, and determining whether a value obtained by subtracting the current most significant bit data from the previous most significant bit data is xe2x80x981xe2x80x99.
In the method, determining whether current most significant bit data are included in a first data area of a look-up table includes subtracting the previous most significant bit data from the current most significant bit data if the number of bits of the previous most significant bit data is greater than that of the current most significant bit data, and determining whether a value obtained by subtracting the previous most significant bit data from the current most significant bit data is xe2x80x981xe2x80x99.
In the method, the current most significant bit data are modulated if a subtracted value between the previous least significant bit data and the current least significant bit data is different from the first reference value.
In the method, the first reference value is xe2x80x981xe2x80x99 and the second reference value is a minimum recognizable gray level change by an observer.
In the method, the current most significant bit data are not modulated if a first subtracted value between the previous most significant bit data and the current most significant bit data is the same as the first reference value and if a second subtracted value between the previous least significant bit data and the current least significant bit data is greater than the second reference value.
In another aspect of the present invention, an apparatus for driving a liquid crystal display includes a frame memory delaying each of most significant bit data and least significant bit data included in input data from an input line, a first comparator comparing the most significant bit data from the input line with the delayed most significant bit data to obtain a first difference amount in the number of bits in the most significant bit data and comparing the first difference amount with a first reference value, a second comparator comparing the least significant bit data from the input line with the delayed least significant bit data to obtain a second difference amount in the number of bits in the least significant bit and comparing the second difference amount with a second reference value, and a selector determining whether the most significant bit data from the input line is to be modulated in accordance with the compared results of the first and second comparators.
In another aspect of the present invention, an apparatus for driving a liquid crystal display includes a frame memory delaying each of most significant bit data and least significant bit data included in input data from an input line, a first comparator comparing the most significant bit data from the input line with the delayed most significant bit data to obtain a first difference amount in the number of bits in the most significant bit data and comparing the first difference amount with a first reference value, a second comparator comparing the least significant bit data from the input line with the delayed least significant bit data to obtain a second difference amount in the number of bits in the least significant bit data and comparing the second difference amount with a second reference value, a modulator modulating the input data, and a selector selecting one of the modulated data and the input data from the input line in accordance with the compared results of the first and second comparators.
In the apparatus, the first reference value is xe2x80x981xe2x80x99 and the second reference value is a minimum recognizable gray level change by an observer.
The modulator modulates the most significant bit data using a look-up table.
In the apparatus, the selector selects the modulated input data if the first difference amount is equal to the first reference value and the second amount is smaller than the second reference value.
The selector selects the modulated data if the first difference amount is different from the first reference value.
The selector selects the input data from the input line if the first difference amount is equal to the first reference value and the second difference amount is larger than the second reference value.
The selector includes a gate device performing a logical operation of output signals of the first and second comparators, and a switching device selecting one of an output of the modulator and the input data from the input line in accordance with an output of the gate device.
The driving apparatus further includes a data driver supplying the modulated data and the input data from the input line to a liquid crystal display, a gate driver applying a scanning signal to the liquid crystal display, and a timing controller applying the input data to the input line and controlling the data driver and the gate driver.
In a further aspect of the present invention, an apparatus for driving a liquid crystal display includes a frame memory delaying each of most significant bit data and least significant bit data included in input data from an input line, a first comparator subtracting the most significant bit data from the input line from the delayed most significant bit data and comparing the subtracted value with a first reference value, a second comparator subtracting the delayed least significant bit data from the least significant bit data from the input line and comparing the subtracted value with a second reference value, a first logic device determining whether the most significant bit data inputted from the input line are modulated in accordance with the compared results of the first and second comparators, a third comparator subtracting the delayed most significant bit data from the most significant bit data from the input line and comparing the subtracted value with the first reference value, a fourth comparator subtracting the least significant bit data from the input line from the delayed least significant bit data and comparing the subtracted value with the second reference value, a second logic device determining whether the most significant bit data inputted from the input line is modulated in accordance with the compared results of the third and fourth comparators, a modulator modulating the most significant bit data from the input line in accordance with a change between the most significant bit data inputted from the input line and the delayed most significant bit data, and a selector selecting one of the modulated data and the input data from the input line in accordance with output logical values of the first and second logic devices.
In the driving apparatus, each of the first and second logic devices is an AND gate.
The selector includes an OR gate performing a logical sum operation of the output signals of the first and second logic devices, and a switching device selecting one of an output of the modulator and the input data from the input line under control of the OR gate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.